Coupling transformer for radiofrequency selectors



y Nov. 18, 1947. w. c. MOORE COUPLING TRANSFORMER FOR RADIO-FREQUENCY SELECTORS Filed July l, 1944 Patented Nov. 18, 1941 COUPLING TRANSFORMER FOR RADIO- FREQUENCY SELECTORS William Cullen Moore, Chicago, Ill., assignor to Motorola, Inc., a corporation of Illinois Application July 1, 1944, Serial No. 543,148

1 claim. l

This invention relates to coupling transformers for radio-frequency selectors and more particularly to such transformers of the type used in double-tuned intermediate-frequency selectors and which permit of independent adjustment of the primary and secondary inductors of the transformer and the coefficient of coupling therebetween.

It is now common practice in double-tuned radio-frequency selectors to utilize fixed sealed capacitors for tuning the selectors because of have in general been complicated,V or have afv fected the inductances of the primary and secondary windings, and thus the tuning of their associated circuits, or have been subject to both Aof these disadvantages.

It has also been the practice in construction of transformers of the type described to dispose each of the windings in a magnetic yoke or shell of powdered iron formed of two opposed cupshaped members so that the resultant yoke was split transversely resulting in a minute air gap around the circumference of the shell. This transverse air gap in the yoke is traversed by the entire flux of the coil and it has been found that the reluctance of the yoke and the Q of the coil are subject to wide variations in manufacture, being particularly sensitive to variations in axial pressure on the opposed cups resulting in variations in the air gap which arise in the course of mounting the transformer structure in the apparatus of which it forms a part.

It is an object of the invention therefore toA magnetic yoke and in which there is no transverse division of the yoke across the iiux path of the encased winding.

It is another object of the invention to provide a new and improved coupling transformer for radio-frequency selectors which is of simple construction and by means of which one or more of the disadvantages of the prior art arrangements may be avoided.

In accordance with the invention, in a doubletuned radio-frequencyv selector comprising primary and secondary tuned circuits, a coupling transformer comprises co-axial primary and secondary windings for connection to the primary and secondary circuits, respectively. A ferromagnetic core element is adjustably associated with each of the windings to adjust the tuning of its associated circuit, while resilient means are provided for mechanically biasing the windings to relative axial movement in a given sense, for example, toward mutual separation, and there are also provided means for adjustably limiting the movement of the windings relative to each other under the action of the biasing means without affecting the adjustments of the core elements.

In accordance with the invention, in a doubletuned radio-frequency selector comprising primary and secondary tuned circuits, a coupling transformer comprises co-axial primary and secprovide a new and improved coupling transformer Y kprovide a new and improved couplingtransformer for radio-frequency selectors in which each of the windings is completely encased in a ferroondary windings for connection to the primary and secondary circuits, respectively. The transformer also includes a longitudinally divided ferro-magnetic yoke encasing each of the windings and comprising a pair of integral semi-cylindrical shells and a ferro-magnetic core element adjustably associated with each of the windings.

`For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings while its scope will be pointedout in the appended claim.

Referring now to the drawing:

Fig. 1 is a perspective view, partly broken away. of 'a coupling transformer embodying the invention;

the structure of the transformer of Fig.` 1; while vFig. 2 is a longitudinal cross-sectional view of Fig. 3 is an exploded perspective view of one of the transformer windings and its associated magnetic yoke.

Referring now to the drawings, there is represented a coupling transformer for use in a double-tuned radio-frequency selector comprising primary and secondary tuned circuits. The transformer comprises co-axial primary and secondary windings I and II for connection to the primary and secondary circuits, respectively, The transformer also includes a ferromagnetic yoke surrounding each of the windings. As illustrated, the yokes of the two windings are identical, each comprising an outer shell I2 of channel-shaped cross section and an inner cylinder I3, so that the windings Ill and II are completely enclosed in their magnetic yokes. Preferably each of the shells I2 and cylinders I3 is formed of compressed powdered iron in a binder, as conventionally used in radio-frequency circuits, and is divided longitudinally and comprises a pair ofintegral semi-cylindrical shells I2a, I2b closely encasing the winding and the inner cylinder I3.

The transformer structure also comprises retaining caps I4, I5 at either end of the winding structures, these caps preferably being formed of molded insulation material such as Bakelite. A ferromagnetic core element is also adjustably associated with each of the windings lII) and II, preferably at the ends thereof remote from each other. For example, the caps I4, I5 may be provided with extending necks Ila, I5a, respectively, internally threaded and ferromagnetic core elements I6 and I'I may be cemented, molded as an integral part of, or otherwise mounted in plugs I8 and I9, respectively, of molded insulation material such as Bakelite threaded to engage the internally threaded necks Y Ila and I5a, respectively, of the retaining caps.

The transformer also includes resilient means for mechanically biasing the windings to relative axial movement in a given sense, together with means for adiustably limiting the movement of the windings relative to each other under the action of the biasing means without affecting the adjustments of the core elements. The resilient means may be a spring of glass or other low-conductivity material or, as illustrated, a rubber cushion 20 interposed between the shells I2 of the windings I0 and I I and held under cornpression, whereby it biases the windings apart from each other. The limiting means may comprise a plurality of adjustable clamping bolts 2-I and 22, or equivalent devices, engaging the retaining caps I4 and I5 to compress the cushion 20 and to form a rigid unitary structure and for adjusting the separation of the windings IU, II; that is, for limiting their relative movement under the bias of the compressed rubber cushion 20. Preferably the clamping bolts 2| and 22 are provided with heads 2 la, 22a engaging one of the retaining caps, for example, the cap I5, and spacer nuts 2lb and 22b engaging the other retaining cap I4 and extending therefrom. A shield can 23 for the transformer structure is supported from and secured to the structure by means of machine screws 24 engaging the spacer nuts 2lb, 22h. Preferably, a sleeve 25 of insulation material such as fiber or molded Bakelite is inserted between the yokes I2 and the shield can 23 to insulate the transformer structure from the shield can, The shield can 23 is provided with an aperture 23a registering with the threaded plug I8 to permit adjustment thereof. In the mounting of the transformer structure a similar aperture will be provided in registry in the plug I9 to permit its ready adjustment.v Preferably, the primary windings In and I-I are connected to appropriate fixed capacitors 26 and 21, respectively, to form a radio-frequency selector comprising coupled tuned circuits. The capacitors may well be of the fixed sealed mica type which are desirable because of their unusual stability characteristics, or of the cylindrical .ceramic type which occupies less space and provides a means of temperature compensation. The capacitors 26 and 21 may be individually mounted on either retaining cap, as illustrated, using the cap as a terminal strip, the windings and their associated capacitors being completely enclosed within the shield can 23.

In the operation of the coupling transformer described, the tuning or alignment of each of the circuits is effected by adjustment of the plug IB or I9 and its associated ferromagnetic core IB or I1, as the case may be, Adjustment of either of the cores I6 or I1 is effective to adjust the inductance of its associated winding, and thus the resonant frequency of the circuit of which it forms a part. Due to the fact that the cores I6 and I1 are disposed at the outer ends of the magnetic yokes I2, adjustment of either core affects substantially the inductance of only its associated winding and does not substantially affect the inductance of the other winding or the coeiiicient of coupling between the two windings. Furthermore, adjustment of the clamping bolts 2I and 22 to vary the spacing of the windings I0 and II is effective to vary the mutual inductance, and therefore the coefficient of coupling, between the two windings but does not vary the relative lspacing of each of the core elements I6 and I1 and'their associated windings I0 and II, respectively. By splitting the yokes I2 longitudinally, the junction therebetween is parallel to the magnetic ux of the encased winding and the reluctance of the path is therefore substantially independent of the air gap between the sections of the shell. This results in a uniformity of reluctance and of the Q of the associated winding substantially independent of the procedure in mounting of the transformer structure and in the particular transformer construction described independent of the adjustment of the bolts 2I and 22 for adjusting the spacing between the windings to adjust the mutual inductance and coefficient of coupling thereof. At the same time, the adjustments are simple and can be readily made from the exterior of the transformer structure windings. The structure is a rigid unitary one readily adapted to mounting in a wave signal receiver and to adjustment therein.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed as new is:

In a double-tuned radio-frequency selector comprising primary and secondary tuned circuits, a coupling transformer comprising co-axial primary and secondary inductive elements for connection to primary and secondary circuits, respectively, each of said inductive elements including a winding, a longitudinally divided ferromagnetic yoke enclosing said winding including a pair of integral semi-cylindrical shells, and a ferromagnetic core adjustably associated with said winding to adjust the tuning of its associated circuit, resilient means interposed between the yokes of said inductive elements for mechanically biasing said elements to relative axial Number the action of said biasing means, each ofsaid l 177,842

clamping devices consisting of a bolt and a spacer nut, and a shield for said selector secured to said spacer nuts.

WILLIAM CULLEN MOORE.

REFERENCES CITED The following references are of record in the le oi this patent:

Number UNITED STATES PATENTS A Name Date LaRue et al. Aug. 8, 1939 ySchone et al Oct. 31, 1939 Cole et al. Oct. 17, 1939 Corssiey Mar. 24. 1936 Riepka Sept. 20, 1938 FOREIGN PATENTS Country Date Great Britain Mar. 6, 1935 

